Layer transfer, or wafer bonding, techniques may be used to form the layers of a semiconductor-on-insulator (SOI) wafer having a bulk substrate layer, a buried oxide (BOX) layer, and a semiconductor layer. Integrated circuits (ICs) with active devices and conductive interconnects can then be formed into and onto the semiconductor layer.
An alternative layer transfer process may start with an SOI wafer (having an active device layer already formed), invert the SOI wafer, bond the SOI wafer to a handle wafer, and then remove the bulk substrate and the BOX. In this manner, the active device layer is transferred to the handle wafer. Additionally, the handle wafer may have previously undergone other processing, so that the resulting bonded semiconductor structure may have desirable features that are difficult to achieve in a single wafer subjected to all of the necessary processing steps. For example, the handle wafer may have a substrate with a trap rich layer, so that the resulting semiconductor structure has the trap rich layer between the active device layer and the substrate. However, if a single wafer had been used, and the trap rich layer had been formed in the wafer before the active device layer, then the processing techniques used to form the active device layer could have degraded the trap rich layer. In this example, therefore, the layer transfer process involving separately processed wafers enables a better final product.
In some respects, or for some applications, the development of SOI technology represented an advance over traditional bulk semiconductor processes. However, SOI wafers are generally more complex and expensive than simple bulk semiconductor wafers. Part of the added expense is due to additional processing steps required to produce the SOI wafers before active device fabrication can begin.